I just sat the PR class with Paul and he mentioned a technique to me when I asked this very same questions. He said to put two circular dots on the top of your monitor. These dots should be a certain length away (interoculuar?, perhaps someone can help with this). Basically I think it's pretty close to the distance between your retinas.
Anyway, soften your gaze on those dots and when the two dots become three dots due to the divergence of your eyes, you're in the PhotoFocus state. As for me I don't have much problem getting there but perhaps this will help someone who does.
